ArfGAPs: Not Only for the Termination - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 256

domain, and the ANK repeat domain, while its PH domain-deleted construct
exhibited much less GAP activity (Kam et al. 2000 ).
The BAR domain was originally recognized as a conserved domain in Bin1,
amphiphysins, and the yeast proteins Rvs167p and Rvs161p (David et al. 1994 ).
The crystal structure of the amphiphysin BAR domain revealed a decade later
demonstrated that BAR domains are dimers (Peter et al. 2004 ). The BAR domain
forms a bundle of three

ʱ

-helices, and self-dimerizes to form a crescent-shaped
structure that binds acid phospholipid and other proteins. The inner curved surface
of the BAR domain has a positively charged membrane-binding interface that
interacts with negatively charged membranes, and appears to be involved in the
tubulation of membranes in intracellular trafficking processes (Gallop and
McMahon 2005 ). The BAR domain also contributes to the membrane scission of
budding vesicles (Dawson et al. 2006 ).
The BAR domain of ASAP1, together with the PH domain, dimerizes and binds
to large unilamellar vesicles containing acidic phospholipids. The recombinant
protein composed of the BAR, PH, and ArfGAP domains efficiently induced
tubular structures. Consistently, in vivo studies showed that ASAP1 induces tubular
formation, which was dependent on the presence of the BAR domain, GTP-Arf1,
and phospholipids, similar to those of large unilamellar vesicles (Nie et al. 2006 ).
These results suggest that ASAP1 functions as an effector of Arf1-dependent
membrane bending via its BAR domain.
The BAR domain of ASAP1 also binds to the FIP3 protein, which is a Rab11
effector. Formation of a ternary complex composed of ASAP1, FIP3, and Rab11
was shown to enhance the GAP activity of ASAP1 against Arf1 (Inoue et al. 2008 ).
In the Golgi/trans-Golgi network (TGN)-to-cilia trafficking, ASAP1 binds
GTP-Arf4, and subsequently interacts with FIP3 and Rab11 to form a complex
that initiates membrane curvature and regulates the budding of post-TGN carriers
targeted to primary cilia (Mazelova et al. 2009 ). The BAR domain was also shown
to be involved in the formation of podosomes in NIH3T3 cells (Bharti et al. 2007 ).
The PRR of ASAP1 binds to numerous proteins containing the SH3 domain,
such as Src-family proteins, Crk, CrkL, paxillin, CIN85, CD2AP, cortactin, and
PRKD2, while the SH3 domain of AMAP1 binds to proline-rich motifs of Fak,
Pyk2 and POB1 (Onodera et al. 2005 , 2012 ; Nam et al. 2007 ; Inoue and Randazzo
2007 ).
Phosphorylation of ASAP1 by Src-family kinases was shown to regulate
invadopodia formation in NIH 3T3 cells (Bharti et al. 2007 ). ASAP1 was also
shown to localize to focal adhesions, and to regulate cell migration through
cytoskeleton remodeling (Kondo et al. 2000 ; Randazzo et al. 2000 ). At focal
adhesions, Pyk2, an isoform of FAK, also binds to ASAP1, and phosphorylation
of Tyr308 and Tyr782 of AMAP1 by Pyk2 was shown to reduce its GAP activity
toward Arf1 (Kruljac-Letunic et al. 2003 ).
ASAP1 also seems to be involved in the recycling of EGFR (Kowanetz
et al. 2004 ). The Cbl family of ubiquitin ligases, including Cbl, Cbl-b, and Cbl-3,
are known to be involved in the downregulation of RTKs (Thien and Langdon
2001 ). ASAP1 can bind to CIN85, which binds to Cbls, and it was shown that